Technical Field
The disclosure is related to processes and apparatus for forming conductive films on a workpiece such as a semiconductor wafer, and in particular for measuring and controlling uniformity in such films.
Background Discussion
Semiconductor integrated circuits have conductive structures defined by trenches filled with a metal such as copper deposited on a barrier layer formed on or over the semiconductor or dielectric substrate. Copper migration into the semiconductor or dielectric substrate is prevented by forming the barrier layer of a suitable material such as tantalum nitride or titanium nitride, for example. Prior to filling the trench with copper, a thin copper seed layer is grown on the barrier layer by physical vapor deposition. Thereafter, a thick copper layer is deposited over the copper seed layer by electroplating, the thick copper layer being sufficient to fill the trench.
One problem with such processes is that non-uniformities in the spatial distribution of sheet resistance of the barrier layer and/or of the seed layer lead to corresponding non-uniformities in the spatial distribution of thickness in the copper layer formed by electroplating. Typically, sheet resistance in the underlying seed/barrier layer affects the electro-plating deposition rate and surface compliance.
A related problem is that thickness sensors, such as eddy current (magnetic field loss) measurement sensors are employed to measure thickness, but typically only after completion of the electroplating process forming the thick copper layer. Generally, eddy current sensors are not suitable for reliable and sufficiently accurate thickness measurements of thin layer such as the barrier and seed layers (whose combined thickness is typically on the order of only 50-200 Angstroms). As a result, non-uniformities in copper layer thickness distribution (or electrical properties distribution) are not discovered until later, and the affected product wafers must be discarded. Any remedial measures are taken with reference to the next batch of wafers in the best case.
Another problem is that the electroplating process itself can introduce non-uniformities in thickness distribution. Such non-uniformities are generally not discovered until completion of the electroplating process or at a later process step.
A related problem is that a final overall thickness distribution measurement is required upon completion of the electroplating process. Typically, such a measurement requires an expensive measurement tool, such as an opaque film metrology device employing optical measurements of elastic deformations stimulated by ultrasonic energy. A less costly alternative, such as an eddy current sensor, involves a slower acquisition of thickness distribution requiring discrete measurements at many locations across the wafer surface, using a move-stop-acquire-move sequence. In certain Cu plating processes, there are process failures that exhibit small-area (<10 mm×10 mm) thickness non-uniformity anywhere near the edge of a 300 mm wafer. Detecting such failures requires a high density of measurement locations (>100 sites) along the perimeter of the wafer, decreasing throughput significantly.